Oil country tubular goods (for example, a tubing through which a fluid such as crude oil flows; and a casing which surrounds a tubing), which are used in oil drilling for mining crude oil or gas oil, are generally several tens of meters long and are connected using threaded joints until they have a length to reach an oil well. In the past, the depth of an oil well was 2,000 m to 3,000 m (2,187 yards to 3,280 yards). However, in recent deep oil wells such as undersea oil wells, there are cases where the depth of an oil well reaches a range of 8,000 m to 10,000 m (8,748 yards to 10,936 yards) or deeper.
Depending on the usage environment thereof, a threaded joint for a steel pipe used for connecting oil country tubular goods is affected by a load such as an axial tension force caused by the weight of oil country tubular goods and a threaded joint; a complex pressure such as an internal or external surface pressure; and heat in the ground. Therefore, the threaded joint is required to maintain the airtightness of oil country tubular goods without being damaged even in such a tough environment.
FIG. 1 is a diagram schematically illustrating an assembly state of oil country tubular goods A and a coupling B which is a threaded joint component. In the oil country tubular goods A, a pin 1 having male threads 11 (refer to FIG. 2) is formed on external surfaces of both ends thereof. In the coupling B, a box 2 having female threads 21 (refer to FIG. 2) is formed on internal surfaces of both ends thereof. An end of the oil country tubular goods A is connected to an end of the coupling B in advance.
FIG. 2 is a diagram schematically illustrating the details of a pin-box structure of a typical threaded joint (also referred to as a special threaded joint) used for connecting the oil country tubular goods A. The pin 1 is a joint component having the male threads 11 and is typically formed on an end portion of the oil country tubular goods A. The box 2 is a joint component having the female threads 21 and is typically formed on an internal surface of the threaded joint component (coupling B). The pin 1 further includes, in addition to the male threads 11, a seal portion 13 and a shoulder portion 14 which are positioned closer to a tip end side of a steel pipe than the threaded portion. Correspondingly, the box 2 further includes, in addition to the female threads 21, a seal portion 23 which is positioned inside of the joint portion; and a shoulder portion 24 which comes into contact with the shoulder portion 14 of the pin 1. The seal portions 13 and 23 and the shoulder portions 14 and 24 of the pin 1 and the box 2 are an unthreaded metal contact portion of the threaded joint for a steel pipe. Reference numeral 12 represents a lip portion at a tip end of the pin 1 and reference numeral 31 represents a portion where the pin 1 is not in contact with the box 2 (a portion in which bleeding oil remains during connecting).
An end of the oil country tubular goods A is inserted into the coupling B, and the male threads 11 and the female threads 21 are connected to each other. As a result, the seal portion 13 of the pin 1 and the seal portion 23 of the box 2 come into contact with each other at an appropriate interference amount to form a metal seal. This metal seal secures the airtightness of the oil country tubular goods A connected to the coupling B. In this kind of threaded joint, in particular, in the seal portions 13 and 23, non-repairable seizure called galling may occur.
When the tubing and casing are lowered to an oil well, there are cases where a threaded joint connected thereto is disconnected due to various kinds of problems; they are pulled up from the oil well; and then they are connected and lowered again. American Petroleum Institute (API) requires galling resistance and airtightness in the sense that galling does not occur and the airtightness of oil country tubular goods is secured even when makeup (connecting) and breakout (disconnecting) are performed 10 times in the case of a tubing joint or 3 times in the case of a casing joint.
In order to improve airtightness and galling resistance during connecting, in the related art, a viscous liquid lubricant (referred to as “compound grease”) containing heavy metal powder is coated onto a contact surface (the threaded portion and unthreaded metal contact portion) of the threaded joint. In API standard BUL5A2, such a compound grease is specified. The compound grease also exhibits rust prevention performance of preventing rust on the coated contact surface.
However, since the compound grease contains a large amount of powder of heavy metals such as zinc, lead, and copper, the coated grease may be washed and overflow on an external surface during the connecting of the threaded joint. As a result, there is a possibility that, in particular, harmful heavy metals such as lead may have an adverse effect on the environment (in particular, marine organisms). In addition, since the work environment deteriorates due to the coating operation of the compound grease, there is a concern of an adverse effect on a human body by the coating operation. Therefore, a threaded joint for a steel pipe capable of exhibiting superior galling resistance without using the compound grease is required.
As a threaded joint which can be used for connecting oil country tubular goods without coating the compound grease, many threaded joints for a steel pipe have been proposed until now in which a contact surface thereof is coated with a solid lubricant coating film composed of an organic or inorganic resin containing lubricating powder such as molybdenum disulfide or graphite.
As described above, in a threaded joint for a steel pipe having a seal portion, since galling is likely to occur particularly in the seal portion, it is necessary that rusting of the seal portion be prevented. This is because galling is likely to occur when the rust occurs. In addition, changes in the shape of the seal portion by corrosion causes deterioration in airtightness. In a threaded joint for a steel pipe which is not coated with the compound grease, since the rust prevention performance by the compound grease is not exhibited, the rust prevention of the seal portion is further important. However, the above-described solid lubricating coating film does not have sufficient rust prevention performance for preventing a cut portion of a joint including a seal portion from being rusted when being stored in a highly corrosive environment for a long period of time.
Patent Document 1 discloses a threaded joint for a steel pipe having superior rust prevention performance that includes an ultraviolet ray-curable resin coating film formed of a photocurable composition, which is curable by the irradiation of ultraviolet rays, on a contact surface of the threaded joint.